The Organization of the Plant Body: Cells, Tissues, and Meristems Chapter 5

Organization of Plant Body Most vascular plants consist of: Shoot System Above ground part Stems, leaves, buds, flowers, fruit Root System Below ground part Main roots and branches

Plant Cells and Tissues Cell wall – surrounds each plant cell Pectin – glues plant cells together Meristems Groups of specialized dividing cells Sources of cells and tissues Not tissues themselves Plant organs – leaves, stems,roots, flower parts

Vascular tissue system Main Tissues of Plants Ground tissue system Most extensive in leaves (mesophyll) and young green stems (pith and cortex) Vascular tissue system Conducting tissues Xylem – distributes water and solutes Phloem – distributes sugars Dermal tissue system Covers and protects plant surfaces – epidermis and periderm

Plant Tissues Simple tissues Composed of mostly one cell type Workhorse cells of plant body Functions Conduct photosynthesis Load materials into and out of vascular system Hold plant upright Store things Help keep plant healthy and functioning

Simple Plant Tissues Tissue type Cell types Parenchyma tissue Parenchyma cells Collenchyma tissue Collenchyma cells Sclerenchyma tissue Fibers, sclereids

Parenchyma Usually spherical or elongated Thin primary cell wall Perform basic metabolic functions of cells Respiration Photosynthesis Storage Secretion

Parenchyma Usually live 1-2 years Crystals of calcium oxalate commonly found in vacuoles May help regulate pH of cells May aggregate to form parenchyma tissue in Cortex and pith of stems Cortex of roots Mesophyll of leaves

Parenchyma Mature cells may be developmentally programmed to form different cell types Wound healing Transfer cells Have numerous cell wall ingrowths Improve transport of water and minerals over short distances At ends of vascular cells help load and unload sugars and other substances

Collenchyma Specialized to support young stems and leaf petioles Often outermost cells of cortex Elongated cells Often contain chloroplasts Living at maturity

Collenchyma Walls composed of alternating layers of pectin and cellulose Can occur as aggregates forming collenchyma tissue Form cylinder surrounding stem Form strands Make up ridges of celery stalk

Sclerenchyma Rigid cell walls Function to support weight of plant organs Two types of cells Fibers Sclereids Both fibers and sclereids have thick, lignified secondary cell walls Both fibers and sclereids are dead at maturity

Sclerenchyma Fibers Long, narrow cells with thick, pitted cell walls and tapered ends Sometimes elastic (can snap back to original length)

Sclerenchyma Fibers Arrangements Aggregates that form continuous cylinder around stems May connect end to end forming multicellular strands May appear as individual cells or small groups of cells in vascular tissues

Sclerenchyma Sclereids Many different shapes Usually occur in small clusters or solitary cells Cell walls often thicker than walls of fibers Sometimes occur as sheets Hard outer layer of some seed coats

Complex Tissues Composed of groups of different cell types Xylem Vessel member, tracheid, fiber, parenchyma cell Phloem Sieve-tube member, sieve cell, companion cell, albuminous cell, fiber, sclereid, parenchyma cell Epidermis Guard cell, epidermal cell, subsidiary cell, trichome (hair) Periderm Phellem (cork) cell, phelloderm cell Secretory structures Trichome, laticifer

The Vascular System

Xylem Complex tissue Transports water and dissolved minerals Locations of primary xylem In vascular bundles of leaves and young stems At or near center of young root (vascular cylinder)

Xylem Cell Types Cell Type Description Trachery element (tracheids and vessel members) Water conducting cells Not living at maturity Before cell dies, cell wall becomes thickened with cellulose and lignin Fibers Strength and support Parenchyma cells Help load minerals in and out of vessel members and tracheids Only living cells found in xylem

Xylem Secondary xylem Forms later in development of stems and roots Water exchanged between cells through tiny openings called pits Simple pits Occur in secondary walls of fibers and lignified parenchyma cells Bordered pits Occur in tracheids, vessel members, and some fibers

Phloem Complex tissue Transports sugar through plant Primary phloem In vascular bundles near primary xylem in young stems In vascular cylinder in roots

Phloem Cell types in angiosperm phloem Sieve-tube members Companion cells Parenchyma Fibers and/or sclereids

Phloem Sieve-tube members Conducting elements of phloem Join end-to-end to form long sieve tubes Mature cell contains mass of dense material called P-protein May help move materials through sieve tubes Usually live and function from 1 to 3 years

Phloem Sieve-tube members mature sieve-tube members have aggregates of small pores called sieve areas One or more sieve areas on end wall of sieve-tube member called a sieve-plate Callose (carbohydrate) surrounds margins of pores Forms rapidly in response to aging, wounding, other stresses May limit loss of cell sap from injured cells

Phloem Companion cells Connected by plasmodesmata to mature sieve-tube member Contain nucleus and organelles Thought to regulate metabolism of adjacent sieve-tube member Play role in mechanism of loading and unloading phloem

Phloem Parenchyma Usually living Function in loading and unloading phloem

Phloem Fibers and/or sclereids Long tapered cells Lignified cell walls

Phloem Gymnosperms and ferns Sieve cells instead of sieve-tube members Conducting elements in phloem Long cells with tapered ends Sieve areas but no sieve plates Usually lack nuclei at maturity Albuminous cells Adjacent to sieve cells Short, living cells Act as companion cells to sieve cells

The Outer Covering of the Plant

Epidermis Outer covering Usually one cell layer thick Functions Epidermis of succulents may be 5-6 cell layers thick Functions Protects inner tissues from drying and from infection by some pathogens Regulates movement of water and gases out of and into plant

Epidermis Cell types Epidermal cells Guard cells Trichomes (hairs)

Epidermis Epidermal cells Main cell type making up epidermis Living, lack chloroplasts Somewhat elongated shape Cell walls with irregular contours Outer wall coated with cutin to form cuticle Cuticle found on all plant parts except tip of shoot apex and root cap Cuticle often very thin in roots

Epidermis Guard cells Found in epidermis of young stems, leaves, flower parts, and some roots Specialized epidermal cells Small opening or pore between each pair of guard cells Allows gases to enter and leave underlying tissue 2 guard cells + pore = 1 stoma (plural, stomata)

Epidermis Guard cells Differ from epidermal cells Crescent shaped Contain chloroplasts

Epidermis Subsidiary cell Forms in close association with guard cells Functions in stomatal opening and closing

Epidermis Trichomes Epidermal outgrowths Single cell or multicellular Example: root hairs Increase root surface area in contact with soil water

Periderm Protective layer that forms in older stems and roots Secondary tissue Several cell layers deep

Periderm Composed of Phellem (cork) Phellogen (cork cambium) On outside Cells dead at maturity Suberin embedded in cell walls Phellogen (cork cambium) Layer of dividing cells Phelloderm Toward inside Parenchyma-like cells Cells live longer than phellem cells

Periderm Secretory structures Primarily occur in leaves and stems May be single-celled or complex multicellular structure Examples Trichomes Could secrete materials out of plant to attract insect pollinators Laticifers Secrete latex which discourages herbivores from eating plant

Meristems

Meristems Special region in plant body where new cells form Area where growth and differentiation are initiated Growth Irreversible increase in size that results from cell division and enlargement Cell differentiation Structural and biochemical changes a cell undergoes in order to perform a specialized function

Meristems Categories of meristems Shoot and apical meristems Ultimate source of all cells in a plant Primary meristems Originate in apical meristems Differentiate into primary tissues Secondary meristems Produce secondary tissues

Root and Apical Meristems RAM – root apical meristem SAM – shoot apical meristem New cells produced by cell division Theoretically could divide forever Does not occur Scarcity of nutrients Branch of plant can only carry so much weight Genetic regulation of growth

Primary Meristems Functions Form primary tissues Elongate root and shoot

Primary Meristems Types of primary meristems Protoderm Procambium Cells differentiate into epidermis Procambium Cells differentiate into primary xylem and primary phloem Ground meristem Differentiates into cells of pith and cortex of stems and roots Differentiates into mesophyll of leaves

Secondary Meristems Functions Cell division Initiation of cell differentiation Lateral growth Increases thickness and circumference of stems and roots

Secondary Meristems Not found in all plants Lacking in plants that grow only one season Leaves usually lack secondary growth Types of secondary meristems Vascular cambium Differentiates into secondary xylem and secondary phloem Cork cambium Differentiates into periderm

Additional Meristems Intercalary meristems Leaf specific meristems In stems Regulates stem elongation Leaf specific meristems Regulates leaf shapes Repair of wounds Formation of buds and roots in unusual places